I don't know why, it's just an overall cool pokemon. It has high defenses and a good amount of health so I can use it as a defensive pokemon and follow up with strong attacks. Also, the rings on its' body give it this spooky look which is perfect for a dark type.

Gyarados is a cool pokemon and all, but I don't agree with it's type. I think it should be a water/dragon type. I just don't see a gyarados flying and it just looks like a dragon, it even learns dragon type moves.